1. Field of the Invention
The present invention relates to acryl-based copolymers and an optical film including the same.
2. Description of the Related Art
Recently, the display technology using various methods such as plasma display panel (PDP), liquid crystal display (LCD), etc., which replace related-art Braun tubes based on the development of optical technologies, has been proposed and put on the market. The higher properties of the polymer material for displays are required. For example, in the case of LCD, realization of wide viewing angle, implementation of high contrast, suppression of change in image color tone according to the viewing angle and uniformity of the screen display have been particularly considered as important problems, while the development toward thin film, lightness, and enlargement of the screen area has been sought.
Accordingly, various polymer films such as polarizing film, polarizer protection film, retardation film, plastic substrate, light guide plate, etc. are used, and as the liquid crystal, various modes of liquid crystal displays such as twisted nematic (TN), super twisted nematic (STN), vertical alignment (VA), in-plane switching (IPS) liquid crystal cells, etc. have been developed. Since all of these liquid crystal cells have intrinsic liquid crystal alignment, they have intrinsic optical anisotropic property, and in order to compensate the optical anisotropic property, a film in which a retardation function is provided by stretching various kinds of polymers has been suggested.
Specifically, since liquid crystal display devices use high birefringence property and alignment of liquid crystal molecules, the refractive indices are different according to the viewing angle and thus the color and brightness of the picture are changed. For example, since most of the liquid crystal molecules that are used in a vertical alignment manner have the thickness refractive index that is larger than the average in-plane refractive index in a liquid crystal display surface, a compensation film in which the thickness refractive index is smaller than the average in-plane refractive index is required in order to compensate this. In addition, light does not pass through the front sides of two polarizing plates that are vertical to each other. However, when the angle is inclined, the light axes of two polarizing plates are not vertical to each other, and thus light leakage occurs. In order to compensate this, a compensation film having the in-plane retardation is required. In addition, display devices using the liquid crystal require both thickness retardation compensation and in-plane retardation compensation in order to widen the angle view.
The retardation compensation films are required to easily control the birefringence. However, the film birefringence is formed not only by a basic birefringence which belongs to the material, but also by the orientation of polymer chains in the film. Most of the orientation of the polymer chains is forcibly performed by force applied from the outside or is caused by the intrinsic properties of the material, and the orientation method of the molecules by the external force is to stretch the polymer film uniaxially or biaxially.
In order to solve the viewing angle problems of LCD due to the intrinsic birefringence properties of liquid crystal, N-TAC, V-TAC, and COP Films have been recently used as compensation or retardation films. However, these films have problems that their prices are high and processes are complicated during manufacturing.